A single switch forward type DC-DC converter is widely used in the middle and low power conversion occasion due to the simplicity of the structure. For the transformer of a forward DC-DC converter, there are many magnetic reset methods, for example RCD reset, third winding reset, LCD reset, resonant reset, and active clamp reset. The resonant reset and active clamp reset are the most common employed methods because they can realize the bi-directional magnetizing of the transformer core, and obtain the duty cycle larger than 0.5. The bi-directional magnetic current is available for improving the anti-saturation of the transformer core and decreasing the transformer size. Furthermore, the duty cycle larger than 0.5 is available for decreasing the voltage stress of the secondary rectifier, and decreasing the filter size and improving the dynamic performance of the converter.
FIG. 1A and FIG. 1B respectively show the configuration and key operation waveforms of a resonant reset forward DC-DC converter of the prior art. The resonant reset forward DC-DC converter is a very simple circuit structure. There is only one main switch S employed in the primary side of the transformer Tr, and a resonant reset capacitor Cr in parallel with the primary winding of the transformer Tr. When the main switch S is turned on, the primary winding of the transformer Tr is connected to the input voltage Vin. The transformer core is magnetized positively and energy is delivered from the source to the load by the transformer coupling. When the main switch S is turned off, the magnetizing current of the transformer Tr charges the resonant capacitor Cr, and the voltage of capacitor Cr increases and resets the transformer core. After a half of resonant period the magnetizing current is reset from the positive maximum value to the negative maximum value, and the voltage of the primary winding remains zero due to the cross conduction of the secondary rectifier. The voltage of the capacitor Cr maintains the same voltage as Vin until the main switch S is turned on again. When the main switch S is turned on, the capacitor Cr is discharged through the main switch S, and the energy stored in the capacitor Cr is dissipated on the main switch S. Thus, the power loss of the main switch S becomes larger. Especially for high frequency and/or high input voltage application, the power loss of the main switch S increases significantly because the energy stored in the capacitor Cr increases with the increase of the switching frequency and the input voltage Vin.
FIG. 2A and FIG. 2B respectively show the configuration and key operation waveforms of the forward DC-DC converter with active clamp reset of the prior art. In this converter the main switch S is employed in the primary side of the transformer Tr, and an active clamp branch including an active switch Sa, an auxiliary diode Da and a clamping capacitor Ca is connected in parallel with the primary winding of the transformer Tr. The auxiliary diode Da is couple across the active switch Sa in parallel. The auxiliary diode Da can also be the parasitic diode of the active switch Sa. An additional driver is employed to keep the main switch S and the active switch Sa conducting complementally. When main switch S is turned on and the active switch Sa is turned off, the primary winding of the transformer Tr is connected to the input voltage Vin. The transformer core is magnetized positively and energy is delivered from the source to the load by the transformer coupling. When the main switch S is turned off and the active switch Sa is turned on, the magnetizing current charges the clamping capacitor Ca. The voltage of the clamping capacitor Ca is coupled across the primary winding of the transformer Tr and resets the transformer Tr. When the active switch Sa is turned off, the magnetizing current is reset from the positive maximum value to the negative maximum value and the clamping capacitor Ca is disconnected from the main switch S. As a result, the energy stored in the clamping capacitor Ca will not dissipate at the time when the main switch S is turned on. Hence, comparing to resonant rest method, the active clamp reset method is a lossless magnetic reset method, but it bears more complicated circuit structure since the active switch Sa needs an additional floating high side driver. Such high side driver needs to float hundreds of voltage, so it increases the converter cost greatly.
Therefore, a demand still exists to provide a new and improved power converter topology that combines the advantages of the resonant reset forward converter and the active clamp forward converter, but overcomes the disadvantages of the prior art.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.